1. Field of the Invention
The present invention relates to a magnetic disk drive used to store information in a computer such as a large-scale computer, a personal computer, a lap-top type computer, etc..
2. Description of the Related Art
A magnetic disk drive, for example, a hard disk drive (hereafter referred to "HDD"), generally comprises a spindle motor for rotationally driving a magnetic disk serving as a magnetic recording medium, and a carriage supporting a magnetic head. As the carriage is swung by means of a voice coil motor (hereafter referred to as "VCM"), the magnetic head is moved over the disk in substantially its radial direction and positioned on a predetermined track of the disk.
The VCM for swinging the carriage comprises upper and lower yokes, which face each other with a distance therebetween. A magnet is fixed to one of the yokes. A voice coil, fixed to the carriage, is interposed between the upper and lower yokes. When the voice coil is excited, the carriage is swung in a predetermined direction by interaction between the magnetic field generated by the voice coil and the magnetic field generated by the magnet.
A magnetic disk has a data zone and a CSS (contact-start-stop) zone, which is an innermost area of the disk, independent of the data zone. The CSS zone is provided to prevent the data zone from being damaged in a nonoperating state of the HDD and in the starting or stopping of the spindle motor. Specifically, when the power is turned off (when the disk is stopped), the magnetic head is brought into contact with the CSS zone of the disk and stopped. When the power is turned on, with the result that the rotation speed of the disk is increased, the magnetic head flies over the disk and is moved by the VCM toward the outer circumference of the disk, i.e., to the data zone.
In the state of the power being off, it is possible that the magnetic head may slide on the surface of the disk due to, for example, some vibration or shock, in which case the disk surface may be damaged by the magnetic head. To prevent this, the HDD comprises a lock mechanism for locking the carriage to hold the magnetic head in a predetermined position, i.e., the CSS zone. As an example of the lock mechanism, U.S. Pat. No. 5,023,736 discloses a magnetic carriage lock mechanism which is provided with a locking magnet and a magnetic member fixed to the carriage and attracted to the locking magnet so that the carriage does not move.
Two torque characteristics are indispensable to the above lock mechanism: one being a large locking torque; and the other a small offset torque. (The locking torque is a holding force for locking the carriage in a position where the magnetic head is situated on the CSS zone. The offset torque is an external force which the magnetic member receives from the VCM magnet, the locking magnet and the like, while the carriage is being swung).
To accurately control the operation of the carriage, it is desirable that the offset torque be kept as small and constant as possible. Therefore, to keep the attractive force of the VCM magnet which acts on the magnetic member constant, the outer peripheral edge of the VCM magnet is formed in a circular arc coaxial with the pivotal center of the carriage, so that the distance between the magnetic member and the VCM magnet is kept constant, independent of the swung position of the carriage.
Recently, a demand, for an increase in the capacity of the HDD as described above, has arisen. To increase the capacity of the HDD, it is proposed that the number of magnetic recording media be increased.
In this case, however, the number of magnetic heads should be increased in accordance with the number of magnetic recording media. Accordingly, the moment of inertia of the carriage is also increased. Hence, to obtain a desired seek performance, it is necessary to increase the driving torque of the VCM to a level corresponding to the increased moment of inertia of the carriage.
The driving torque of the VCM can be increased by increasing the thickness of the magnet, without changing the plane configuration, to increase the amount of flux. However, in this case, since the diffusion of flux in a peripheral portion of the magnet is increased in accordance with an increase in the thickness, the linearity of the driving torque is undesirably lowered.
The lowering of linearity of the driving torque means an increase in the difference between the torque obtained when the voice coil is positioned at a central portion of the magnet in its circumferential direction (the direction of the movement of the coil) and the torque obtained when the voice coil is positioned in each end portion of the magnet with respect to the circumferential direction. When the linearity of the driving torque of the VCM is low, it is difficult to accurately control the movement of the carriage, i.e., the positioning of the magnetic head.